1. Field of the Invention
This invention relates to a solar module, and more particularly to a construction of a solar module and a material used as a coating or adhesive material.
2. Description of the Prior Art
Most of the conventional solar modules comprise crystalline-type cells such as single crystal silicon cells and polycrystal silicon cells. A comprehensive discussion of available encapsulation techniques and data for silicon flat-plate solar modules is presented, for example, in "Photovoltaic-Module Encapsulation Design and Materials Selection: Volume 1" by E. Cuddihy, W. Carroll, C. Coulbert, A. Gupta and R. Liang, by Jet Propulsion Laboratory (JPL) (June 1, 1982). A few of the techniques are applicable to modules comprising thin film-type solar cells but most are not. Therefore, it has been desired to develop a new technique for modules comprising thin film-type solar cells.
In the conventional module comprising crystalline-type solar cells, photovoltaic parts and connections thereof are buried or sandwiched in pottants made of such materials as ethylene vinyl acetate (EVA), ethylene methyl acrylate (EMA), acrylic laminating film, poly-n-butyl acrylate (InBA), aliphatic polyester urethane, silicone elastomer, and polyvinyl butyral (PVB). The pottants must be transparent, chemically stable, processible, elastomeric, and commercially available. The pottants must have electrical insulation and inherent weatherability (retention of transparency and mechanical integrity). However, the above listed pottants are not integral, and they often surrender to oxidation and hydrolysis.
In the structure of a module where crystalline-type cells are buried in a pottant or sandwiched between two sheets of a transparent thermoplastic, the layer of plastics is thick, resulting in an extensive, easy oxidation and hydrolysis of plastics and buried or sandwiched parts when the layer is directly exposed to oxygen and water (or water vapor).
In the case of the thin-film type cells, the photovoltaic elements are formed on a substrate of a small area, usually less than 100 cm.sup.2, because it is technically difficult to form a thin film of a large area. Such cells are coated with a resin, for example, epoxy or acrylic resin and used as a small power source for devices such as a wrist watch, a game machine, and a hand calculator. When used as a large power source, the thin-film type cells are connected in series or parallel and then buried or sandwiched between plastic sheets in the same way as the crystalline type cells as described above. Kinds of resins that have been used as pottants, adhesives, or coating materials are: EVA, EMA, acrylics, aliphatic polyester urethane, silicon elastomer, PVB, epoxys, etc.
FIG. 1 is a principal cross-sectional view of a conventional solar module. Solar cells 1 are connected by lead wires 2 and then buried in a transparent pottant 3. The potted solar cells are sandwiched between an upper reinforced glass 4 and a lower protecting backcover. The solar cells 1 are made of sliced wafers of a single-crystal silicon, and therefore are thick. Lead wires 2 each connecting an upper side of a cell with a lower side of another cell are also bulky. Consequently, the thickness of the transparent pottant burying solar cells 1 and lead wires 2 cannot be thin but is considerably thick. At an early stage, the transparent pottant was made of a thermosetting silicone resin. But recently, a thermoplastic such as PVB and EVA has been used. Regardless of the kind of pottant, the thickness of the pottant is considerably large. Due to the large thickness and lack of ability to prevent permeation of water, water permeates into the solar module and degrades the solar cells.
As described above, the conventional construction of solar modules is expensive and heavy. Therefore, it has been desired to develop a new construction of solar module which is simple in construction, light in weight and inexpensive, but stable in performance.